The role of free radical chemistry in many biological processes in becoming increasingly recognized. There are at present, however, no satisfactory methods for studying the distribution of radical content on bioligical macromolecules. The present effort may afford a means for accomplishing this objective. Thus, an aqueous solution of the macromolecular sample (presently studied are lysozyme and ribonuclease) is Gamma-irradiated in the presence of a spin-trapping reagent to stabilize the trasient free radical content that results from irradiation. The sample is degraded proteolytically, under conditions that ensure maintenance of the radical content. The resulting amino acids (and possibly small peptides) may then be separated, identified, and radical content of each component studied by electron spin resonance (e.s.r.). Initial efforts in producing digests, in which the radical content has been maintained, have proven successful. Further efforts will be directed toward separation and e.s.r. study of the radical-containing components.